As semi-conductor processes continue to scale, the number of transistors available in a silicon-chip increases. Integrating the whole system on a chip becomes a possibility for many applications. However, most systems are composed of many separate components such as DRAM, EEPROM, and FLASH which are fabricated using specialized processes. These processes may not be compatible with the common logic process used for manufacturing logic components such as micro-processors and system logic.
Common logic or application-specific-integrated-circuit (ASIC) processes are typically the most aggressively scaled processes offered by silicon foundries. In order to integrate specialized memory components (e.g., EEPROM, DRAM and FLASH) on the same substrate as logic components, processing steps may need to be added to a logic process. However, adding steps to logic manufacturing processes adds to the manufacturing cost of all the components of the chip, including the logic components that wouldn't otherwise use those steps.
Alternatively, memory circuits can be designed to be compatible with the logic process. However, the memory cell area fabricated from the logic process is generally bigger, e.g., three times or more, than the area of memory cell fabricated with added processing. Therefore, it is desirable to fabricate non-volatile memory for embedded in logic process with minimum additional processing steps to reduce cell area without incurring much additional cost to the logic process.
The common logic processes offered by silicon foundries with feature sizes in a very deep submicron (e.g., 130 nm and 90 nm) area may include the following characteristics: 1) single-layer poly-silicon (“single-poly); and 2) transistors with thin oxide for the logic circuits operating at a lower supply voltage (e.g., 1.0 v) and transistors with thick oxide for handling higher IO voltages (for example 3.3 v).
However, non-volatile memory can require voltage levels higher than those used by traditional transistors fabricated using a generally available logic process. High voltage transistors can be fabricated using a specialized logic process, but this requires extra cost and steps. Therefore the non-volatile memories are not suitable for embedded applications using commonly available logic processes.